MONDAY, March 17, 2014 (HealthDay News) -- Having a lower IQ or poorer fitness at age 18 might increase a man's risk of developing dementia before age 60, a new study suggests.

The analysis of data from 1.1 million Swedish men suggested that the risk of early onset dementia was 2.5 times higher in those with poorer heart fitness, four times higher in those with a lower IQ and seven times higher in those with both risk factors.

The men were first tested as part of Sweden's national military service conscription and followed for up to 42 years.

The increased dementia risk remained even when the University of Gothenburg researchers took into account other risk factors, such as socioeconomic status and medical and family history, according to the study, which was published online recently in the journal Brain.

"Previous studies have shown the correlation between cardiovascular fitness and the risk of dementia in old age," study leader Jenny Nyberg said in a university news release. "Now, for the first time, we can show that the increased risk also applies to early onset dementia and its precursors."

Although the study found an increased risk of early dementia among men who had lower IQs or worse fitness tests in their teens, it did not prove a cause-and-effect relationship.

Study senior author Professor Georg Kuhn said it was already known that both physical and mental exercise reduce the risk of brain diseases.

"Physical exercise increases nerve cell complexity and function, and even generation of new nerve cells in the adult brain, which strengthens our mental and physiological functions," Kuhn said. "In other words, good cardiovascular fitness makes the brain more resistant to damage and disease."

The researchers said many people who develop early onset dementia are of working age and might still have children living at home. This means the impact of the disease on these patients and their families can be more devastating than among those who develop dementia at an older age.

Even so, people with early onset dementia tend to be overlooked, the study authors said.

"This makes it important to initiate more research into how physical and mental exercise can affect the prevalence of different types of dementia," Nyberg said. "Perhaps exercise can be used as both a [preventive] and a treatment for those in the risk zone for early onset dementia."